Polycarbonate resin molding material for optical use

ABSTRACT

A polycarbonate resin molding material for optical use which is pellets formed from a polycarbonate resin and having a bulk density of 0.72 kg/l or more and an optical disk substrate formed therefrom. 
     According to the present invention, use of the above pellets makes it possible to obtain an optical disk substrate which rarely experiences the production of a silver streak, reduce the molding cycle and smoothen the molding operation.

FIELD OF THE INVENTION

The present invention relates to a molding material for optical use, forexample, a resin molding material suitable for the manufacture of anoptical recording medium for recording various information signals suchas voice signals or image signals and to a substrate obtained therefrom.

DESCRIPTION OF THE PRIOR ART

Optical disks for recording and reproducing information through exposureto a laser beam, such as digital audio disks (so-called compact disks),optical video disks (so-called laser disks), recordable disks,opto-magnetic disks and phase-change disks have been implemented.

Out of these, the compact disks and the laser disks are read-only memory(ROM) type optical disks. Generally these optical disks have pitscorresponding to information signals on a transparent resin substrateand an aluminum (Al) reflection layer as thick as 40 nm or more formedon the substrate. In these optical disks, a change in reflectance causedby an optical interference produced by a pit is detected to reproduce aninformation signal.

Meanwhile, the recordable optical disks are optical disks to whichdesired information can be written by a user and the opto-magnetic disksand the phase-change type disks are RAM (random access memory) typeoptical disks to which desired information can be written repeatedly.

That is, the recordable optical disks comprise a transparent resinsubstrate and a recordable recording layer thereon whose opticalproperties are changed irreversibly by exposure to a laser beam or whosesurface becomes uneven by exposure to a laser beam. This recording layeris made from a cyanine-based, phthalocyanine-based or azo-based organicdye which is decomposed by heat from exposure to a laser beam to changeits optical constant and causes the deformation of the substrate throughits volume change.

The opto-magnetic disks are rewritable optical disks in whichinformation can be written and erased by a user repeatedly and whichcomprise a vertically magnetized layer having a magneto-optical effect(for example, Kerr effect), such as a Tb—Fe—Co amorphous alloy thinfilm, formed on a transparent resin substrate. A recording pit is formedin this opto-magnetic disk by magnetizing a micro-area corresponding toan information signal of the vertically magnetized layer in an upwarddirection or a downward direction. The information signal is reproduced,making use of the fact that the rotation angle θk (Kerr rotation angle)of linear polarization of reflected light differs according to themagnetization direction of the vertically magnetized layer.

The phase-change disks are rewritable disks like the opto-magnetic disksand a Ge—Sb—Te phase-change material which is initially crystalline andbecomes amorphous upon exposure to a laser beam is used therein, forexample. In this recording layer, a recording pit is formed by changingthe phase of a micro-area corresponding to an information signal and thedifference in reflectance between an amorphous portion corresponding tothe pit and other crystalline portion is detected to reproduce theinformation signal.

The above opto-magnetic disks and phase-change disks have a four-layerstructure consisting of a recording layer, transparent dielectric layerssandwiching the recording layer and an aluminum (Al) reflection layerformed thereon in order to prevent the oxidation of the recording layerand increase the modulation degree of a signal by multiple interferencein most cases. The dielectric layers are a silicon nitride layer orZn—SiO₂ mixed film.

Studies are being made energetically to use the above optical disks forrecording digital images. A digital versatile disk (DVD) has beendeveloped as such an optical disk.

This DVD has the same diameter of 120 mm as CD and is designed to recordimage information equivalent to one movie and reproduce high-qualityimage information equivalent to that of the current TV.

A recording capacity 6 to 8 times greater than CD is required to recordsuch image information on an optical disk. Therefore, the wavelength ofa laser beam is reduced to 635 to 650 nm in DVD compared with 780 nm ofCD and the numerical aperture NA of an objective lens is increased to0.52 or 0.6 in DVD compared with 0.45 of CD in order to reduce the trackpitch and the shortest recording mark length of pits, thereby raisingrecording density.

Out of these, an increase in the numeral aperture NA of the object lensreduces the warp tolerance of a disk substrate. The thickness of thesubstrate of DVD is reduced to 0.6 mm which is much smaller than 1.2 mmof CD in order to shorten the distance of a laser beam passing throughthe disk substrate so as to compensate for a reduction in the warptolerance. To compensate for a reduction in the strength of the diskresulted by the reduced thickness of the substrate, as described in JP-A6-274940 (the term “JP-A” as used herein means an “unexamined publishedJapanese patent application”), a bonding-together structure that anothersubstrate is bonded on the recording layer formed on the substrate isadopted. The recording layer of the laminated optical disk may be a ROMtype recording layer, recordable type recording layer or RAM typerecording layer which is used in the above single-substrate structure.

Further, the bonded optical disk is available in a one-side bondedoptical disk in which only one side is used and both-side bonded opticaldisk in which both sides are used.

A polycarbonate resin which is excellent in moldability, strength, lighttransmission and humidity resistance is widely used in the above opticaldisk resin substrates.

Since an optical disk makes use of micro-irregularities formed on aresin substrate to record or reproduce information by a laser beam, whena defect existent in the resin substrate is larger than eachirregularity, it has a great influence on the reliability of informationrecording or reproduction. Therefore, the production of a silver streakwhich is such a defect must be suppressed.

The causes of producing a silver streak include the hydrolysis of aresin pellet due to incomplete drying, thermal decomposition in acylinder and the inclusion of air from a hopper side. As means ofsuppressing the production of a silver streak caused by the inclusion ofair, it has been proposed to limit the length of a pellet (JP-A 7-52272)and to limit the length and long diameter of a pellet (JP-A 11-35692).However, no explanation is given of the bulk density of pellets in theseproposals and the bulk density cannot be set to a suitable range simplyby limiting the length and long diameter of each pellet in theseproposals.

Problem to be Solved by the Invention

The present invention has been made in view of the above problems andintensive studies have been made on the problems. As a result, it hasbeen found that the production of a silver streak is suppressed bysetting the bulk density value of pellets to a certain range and thatuse of the pellets can reduce the metering time, can shorten the moldingcycle, prevents a pellet feed trouble and smoothen molding operation.The present invention has been accomplished by these findings.

Means for Solving the Problems

According to the present invention, there are provided a polycarbonateresin molding material for optical use which is pellets formed from apolycarbonate resin and having (1) a bulk density of 0.72 kg/l (liter)or more, and an optical disk substrate formed from the material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the present invention, a detailed description is given of pellets ofan aromatic polycarbonate resin as a molding material suitable formolding a substrate for improving the reliability of informationrecording and reproduction as an optical disk substrate for digitalversatile disks (DVD) typified by CD-R, CD-RW, MO, DVD-ROM, DVD-Audio,DVD-R and DVD-RAM, particularly a high-density optical disk substratefor DVD.

The polycarbonate resin used in the present invention is generallyobtained by reacting a diphenol with a carbonate precursor by aninterfacial polymerization or ester exchange (melt polymerization)method. Typical examples of the diphenol used include hydroquinone,resorcinol, 4,4′-dihydroxydiphenyl, bis(4-hydroxyphenyl)methane,bis{(4-hydroxy-3,5-dimethyl)phenyl}methane,1,1-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane,2,2-bis(4-hydroxyphenyl)propane (generally called bisphenol A),2,2-bis{(4-hydroxy-3-methyl)phenyl}propane,2,2-bis{(4-hydroxy-3,5-dimethyl)phenyl}propane,2,2-bis{(3,5-dibromo-4-hydroxy)phenyl}propane,2,2-bis{((3-isopropyl-4-hydroxy)phenyl}propane,2,2-bis{(4-hydroxy-3-phenyl)phenyl}propane,2,2-bis(4-hydroxyphenyl)butane, 2,2-bis(4-hydroxyphenyl)-3-methylbutane,2,2-bis(4-hydroxyphenyl)-3,3-dimethylbutane,2,4-bis(4-hydroxyphenyl)-2-methylbutane,2,2-bis(4-hydroxyphenyl)pentane,2,2-bis(4-hydroxyphenyl)-4-methylpentane,1,1-bis(4-hydroxyphenyl)cyclohexane,1,1-bis(4-hydroxyphenyl)-4-isopropylcyclohexane,1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane,9,9-bis(4-hydroxyphenyl)fluorene,9,9-bis{(4-hydroxy-3-methyl)phenyl}fluorene,α,α′-bis(4-hydroxyphenyl)-o-diisopropylbenzene,α,α′-bis(4-hydroxyphenyl)-m-diisopropylbenzene,α,α′-bis(4-hydroxyphenyl)-p-diisopropylbenzene,1,3-bis(4-hydroxyphenyl)-5,7-dimethyladamantane, 4,4′-dihydroxydiphenylsulfone, 4,4′-dihydroxydiphenyl sulfoxide, 4,4′-dihydroxydiphenylsulfide, 4,4′-dihydroxydiphenyl ketone, 4,4′-dihydroxydiphenyl ether and4,4′-dihydroxydiphenyl ester. They may be used alone or in admixture oftwo or more.

Out of these, homopolymers and copolymers obtained from at least onebisphenol selected from the group consisting of bisphenol A,2,2-bis{(4-hydroxy-3-methyl)phenyl}propane,2,2-bis(4-hydroxyphenyl)butane, 2,2-bis(4-hydroxyphenyl)-3-methylbutane,2,2-bis(4-hydroxyphenyl)-3,3-dimethylbutane,2,2-bis(4-hydroxyphenyl)-4-methylpentane,1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane andα,α′-bis(4-hydroxyphenyl)-m-diisopropylbenzene are preferred, and ahomopolymer of bisphenol A and a copolymer of1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane and bisphenol A,2,2-bis{(4-hydroxy-3-methyl)phenyl}propane orα,α′-bis(4-hydroxyphenyl)-m-diisopropylbenzene are particularlypreferred. Particularly, a homopolymer of bisphenol A has an excellentutility value.

The carbonate precursor is a carbonyl halide, carbonate ester,haloformate or the like. Specific examples of the carbonate precursorinclude phosgene, diphenyl carbonates and dihaloformates of diphenols.

A catalyst and an antioxidant for the diphenol may be used to produce apolycarbonate resin by reacting the above diphenol with the abovecarbonate precursor by the interfacial polymerization or ester exchangemethod. The polycarbonate resin may be a branched polycarbonate resinobtained by copolymerizing a polyfunctional aromatic compound having afunctionality of 3 or more, a polyester carbonate resin obtained bycopolymerizing an aromatic or aliphatic difunctional carboxylic acid, ora mixture of two or more of the obtained polycarbonate resins.

The reaction which is carried out by the interfacial polymerizationmethod is generally a reaction between a diphenol and phosgene in thepresence of an acid binder and an organic solvent. The acid binder isselected from an alkali metal hydroxide such as sodium hydroxide orpotassium hydroxide and an amine compound such as pyridine. The organicsolvent is a hydrocarbon halide such as methylene chloride orchlorobenzene. To promote the reaction, a catalyst selected from atertiary amine, quaternary ammonium compound and quaternary phosphoniumcompound such as triethylamine, tetra-n-butylammonium bromide ortetra-n-butylphosphonium bromide may be used. The reaction temperatureis generally 0 to 40° C., the reaction time is 10 minutes to 5 hours andpH during the reaction is preferably maintained at 9 or more.

The reaction which is carried out by the ester exchange method isgenerally a reaction between a diphenol and a carbonate ester which iscarried out by mixing together the diphenol and carbonate ester underheating in the presence of an inert gas and distilling out the formedmonohydroxy compound (such as phenol). The reaction temperature whichdiffers according to the boiling point of the formed monohydroxycompound is generally 120 to 350° C. The pressure of the reaction systemis reduced to 10 to 0.1 Torr in the latter stage of the reaction to makeit easy to distill out the formed monohydroxy compound. Since themonohydroxy compound formed during the reaction remains in thepolycarbonate resin, a sufficiently reaction time is necessary,specifically 1 to 4 hours.

The carbonate ester is an ester such as an allyl group having 6 to 10carbon atoms, aralkyl group or alkyl group having 1 to 4 carbon atomsall of which may be substituted. Specific examples of the carbonateester include diphenyl carbonate, ditolyl carbonate,bis(chlorophenyl)carbonate, m-cresyl carbonate, dinaphthyl carbonate,bis(diphenyl)carbonate, dimethyl carbonate, diethyl carbonate anddibutyl carbonate. Out of these, diphenyl carbonate is preferred.

A polymerization catalyst may be used to accelerate the polymerizationspeed. Examples of the polymerization catalyst include alkali metalcompounds such as sodium hydroxide, potassium hydroxide and sodium saltsand potassium salts of diphenols, alkali earth metal compounds such ascalcium hydroxide, barium hydroxide and magnesium hydroxide,nitrogen-containing basic compounds such as tetramethylammoniumhydroxide, tetraethylammonium hydroxide, trimethylamine andtriethylamine, alkoxides of alkali metals and alkali earth metals,organic acid salts of alkali metals and alkali earth metals, zinccompounds, boron compounds, aluminum compounds, silicon compounds,germanium compounds, organic tin compounds, lead compounds, osmiumcompounds, antimony compounds, manganese compounds, titanium compoundsand zirconium compounds all of which are generally used foresterification reaction and ester exchange reaction. The above catalystsmay be used alone or in combination of two or more. The amount of thepolymerization catalyst is preferably 1×10⁻⁸ to 1×10⁻⁴ equivalent, morepreferably 1×10⁻⁷ to 5×10⁻⁴ equivalent based on 1 mol of the rawmaterial diphenol.

To reduce the number of the phenolic terminal groups in thepolymerization reaction, a terminal capping agent other than amonofunctional phenol, such as bis(chlorophenyl)carbonate,bis(bromophenyl)carbonate, bis(nitrophenyl)carbonate,bis(phenylphenyl)carbonate, chlorophenylphenyl carbonate,bromophenylphenyl carbonate, nitrophenylphenyl carbonate, phenylphenylcarbonate, methoxycarbonylphenylphenyl carbonate orethoxycarbonylphenylphenyl carbonate, is preferably added in the latterstage of the polycondensation reaction or after the end of the reaction.Out of these, 2-chlorophenylphenyl carbonate and2-methoxycarbonylphenylphenyl carbonate are preferred, and2-methoxycarbonylphenylphenyl carbonate is particularly preferred.

The molecular weight of the polycarbonate resin is preferably 10,000 to22,000, more preferably 12,000 to 20,000, particularly preferably 13,000to 18,000 in terms of viscosity average molecular weight (M). Thepolycarbonate resin having the above viscosity average molecular weighthas sufficiently high strength as a material for optical use andexcellent melt fluidity at the time of molding and is free from moldingdistortion. The viscosity average molecular weight in the presentinvention is obtained by inserting a specific viscosity (η_(sp))obtained from a solution containing 0.7 g of a polycarbonate resindissolved in 100 ml of methylene chloride at 20° C. into the followingequation.

η_(sp) /c=[η]+0.45×[η]² c ([η] represents a limiting viscosity)

[η]=1.23×10⁻⁴M^(0.83)

c=0.7

The pellets of the present invention are preferably polycarbonate resinpellets produced by the above ester exchange method and a polycarbonateresin containing a phenolic hydroxyl group (OH group) at a terminal inan amount of 5 to 100 equivalents, preferably 10 to 70 equivalents basedon 1 ton of the resin is advantageous as the polycarbonate resin.

After the polycarbonate resin is produced by a conventionally knownmethod (interfacial polymerization method, ester exchange method, etc.),it is preferred to remove impurities and foreign matter such as a lowmolecular weight component and unreacted components by carrying outalkali extraction or filtration in a solution state, or by washing agranulated resin after granulation (removal of the solvent) with apolycarbonate poor solvent or non-solvent exemplified by a ketone suchas acetone, aliphatic hydrocarbon such as hexane and aromatichydrocarbon such as xylene. In the extrusion step (pelletizing step) forobtaining polycarbonate resin pellets to be injection molded, it ispreferred to remove foreign matter by passing a polycarbonate resinthrough a sintered metal filter having a filtration accuracy of 10 μmwhile it is molten. If necessary, a release agent such as a polyhydricalcohol fatty acid ester, an antioxidant such as phosphorus-basedantioxidant and other additives are preferably added. In either case,the contents of foreign matter, impurities and solvent in the rawmaterial resin before injection molding (before the pelletizing step)must be reduced as much as possible.

Pellets can be produced by extruding the polycarbonate resin into astrand by a melt extruder, passing it into a cooling bath filled withhot water to be cooled and cutting it with a cutter.

The pellets which satisfy conditions specified in the present inventioncan be obtained by selecting means and conditions for the preparation ofthe pellets and optionally suitably carrying out the operation ofselecting the obtained pellets.

That is, the pellets of interest can be obtained by suitably selectingpreparation means and conditions such as the shape of a die hole in themelt extruder, melt extrusion temperature, the amount of extrusion, thetemperature and temperature distribution of cooling water, the take-offtension of the strand; the take-off speed of the strand, the rotationspeed of the cutter, the temperature of the strand at the time ofcutting, the condition of the blade of the cutter and clearance. Sincethe obtained pellets include powders, small pieces and coarse pelletsfor unsuccessful cutting, it is desired to remove these. To this end,the operation of selecting the pellets is desirably carried out toobtain pellets uniform in shape. This selection operation is carried outusing a punching metal or sieve for example.

The pellets of the molding material (aromatic polycarbonate resin) ofthe present invention must have a bulk density of 0.72 kg/l or more.

When the pellets have a bulk density of 0.72 kg/l or more, theproduction of a silver streak can be suppressed, the reliability ofinformation recording can be improved, and the metering time can beshortened. When the bulk density of the above pellets is outside theabove range, a silver streak is produced, the high reliability ofinformation recording and reproduction is not obtained, and the meteringtime cannot be shortened.

The bulk density is 0.72 kg/l or more, preferably 0.73 kg/l or more,more preferably 0.74 kg/l or more because a positive effect can beobtained. The upper limit of bulk density is calculated to be 0.88 kg/lbased on the specific gravity and the closest packing rate of apolycarbonate based on the assumption that the pellets are spherical.However, it changes according to the shape characteristics and grainsize distribution of the pellets. In general, the upper limit of bulkdensity is 0.80 kg/l. The pellets have a circular, oval or rectangularcross section, a length of 2.5 to 3.5 mm and a weight of 8 to 35 mg.

It has been found from researches conducted by the inventors of thepresent invention that when the pellets have a bulk density of 0.72 kg/lor more, the production of a silver streak can be suppressed and thatthe size and ratio of air bubbles contained in the pellets are alsoconnected with the production of a silver streak.

That is, the aromatic polycarbonate resin pellets which are the moldingmaterial of the present invention have an air bubble evolution ratio of35% or less, a maximum air bubble volume of 2.0 mm³ or less and an airbubble volume ratio of 2.0% or less, advantageously.

When the air bubble evolution ratio in the pellets is reduced to 35% orless, the maximum air bubble volume is reduced to 2.0 mm³ or less andthe air bubble volume ratio is reduced to 2.0% or less, the productionof a silver streak is further suppressed and the reliability ofinformation recording can be improved.

The air bubble evolution ratio in the pellets is the ratio of the numberof pellets containing air bubbles (or voids) to the total number ofpellets. The maximum air bubble volume is the largest volume of airbubbles (voids) in one pellet and the air bubble volume ratio is theratio of the total volume of air bubbles (voids) in the pellets to thetotal volume of the pellets. The air bubbles (voids) have a volume of0.01 mm³ or more.

When the pellets have an air bubble evolution ratio of 35% or less,preferably 30% or less, more preferably 25% or less, a maximum airbubble volume of 2.0 mm³ or less, preferably 1.5 mm³ or less, morepreferably 1.0 mm³ or less, and an air bubble volume ratio of 2.0% orless, preferably 1.5% or less, more preferably 1.0% or less, its effectbecomes positive.

It has been found from researches conducted by the inventors of thepresent invention that the average weight value of the pellets and itsstandard deviation have an influence on the manufacture of an opticaldisk substrate which rarely experiences the production of a silverstreak. That is, when the average weight value of the pellets is 13 to26 mg, preferably 14 to 24 mg, more preferably 15 to 20 mg and itsstandard deviation is 2.2 mg or less, preferably 2.0 mg or less, morepreferably 1.8 mg or less, its effect becomes more positive.

Further, researches conducted by the inventors of the present inventionalso have revealed that when the average value of the long diameter toshort diameter ratio (to be referred to as “long diameter/short diameterratio” hereinafter) of the cut section of the pellets is 1.3 to 1.7 andits standard deviation is 0.08 to 0.15, the production of a silverstreak can be further reduced and the operation of molding becomessmoother. The long diameter of the cut section of each pellet is thelength (mm) of the longest diameter of the cut section and the shortdiameter is the length (mm) of the shortest diameter of the cut section.

When the average value of the long diameter/short diameter ratio of thecut section of the pellets is set to 1.3 to 1.7 and its standarddeviation is set to 0.08 to 0.15, the production of a silver streak issuppressed and the reliability of information recording can be improvedin the pellets. When the long diameter/short diameter ratio is outsidethe above range, a silver streak is produced and the high reliability ofinformation recording and reproduction is not obtained.

When the average value of long diameter/short diameter ratio is 1.3 to1.7, preferably 1.4 to 1.6, a more positive effect can be obtained. Whenthe standard deviation of the ratio is 0.08 to 0.15, preferably 0.10 to0.13, a more positive effect can be obtained.

The pellets of the present invention have a repose angle of 23 to 28°,preferably 24 to 27°, more preferably 25 to 26° at the time ofaccumulation because the production of a silver streak can be suppressedmore, the reliability of information recording can be improvedadvantageously, and the operation of molding becomes smooth. The reposeangle of the pellets is a value calculated from the following expressionby dropping the pellets on a 13 cm-diameter polycarbonate disk from aheight of 5 cm to be piled up and measuring the height (H) of theconically piled layer of the pellets.

Repose angle Φ(°)=tan⁻¹(H/6.5)

An injection molding machine (including an injection compression moldingmachine) is used to produce an optical disk substrate from thepolycarbonate resin pellets. A commonly used injection molding machinemay be used but an injection molding machine whose cylinder and screwsare made from a material having low adhesion to resins and corrosionresistance and abrasion resistance is preferably used in order tosuppress a production of carbides and to improve reliability of disksubstrate. Injection molding conditions include a cylinder temperatureof 300 to 400° C. and a mold temperature of 50 to 140° C. and theinjection molding atmosphere is preferably as clean as possible. It isimportant that pellets to be molded should be dried completely so as toremove water and care must be taken to prevent residence which may causethe decomposition of a resin. It is also important that a substratehaving abnormal birefringence and mechanical properties should not beused as a product or test substrate.

An optical disk substrate formed from the polycarbonate resin moldingmaterial for optical use (pellets) of the present invention rarelyexperiences the production of a silver streak and is excellent as asubstrate for optical disks of digital versatile disks (DVD) typified byCD-R, CD-RW, MO, digital video disks, DVD-ROM, DVD-Audio, DVD-R andDVD-RAM, particularly a substrate for DVDs.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a cylindrical air bubble contained in apellet when the pellet is observed from the side (right and left endportions shown on the drawing are cut sections, the same shall apply toFIG. 2 and FIG. 3);

FIG. 2 is a schematic diagram of a semi-cylindrical air bubble containedin a pellet when the pellet is observed from the side; and

FIG. 3 is a schematic diagram of a spherical air bubble contained in apellet when the pellet is observed from the side.

EXPLANATIONS OF SYMBOLS

L: length in a horizonal direction of the cylindrical air bubble(FIG. 1) or the semi-cylindrical air bubble (FIG. 2)

r: length in a vertical direction (diameter) of the cylindrical airbubble (FIG. 1), length in a vertical direction (diameter) at anintermediate point of L of the semi-cylindrical air bubble (FIG. 2) orlength (diameter) of the spherical air bubble (FIG. 3)

EXAMPLES

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting. Examples 1 to 5 and Comparative Examples 1 to 5

Polycarbonate resin pellets in Examples and Comparative Examples werecylindrical pellets having an oval cross section obtained by extruding aresin formed by the ester exchange or interfacial polymerization methodinto a strand by a melt extruder, passing it through a cooling tankfilled with hot water to be cooled and cutting it with a cutter. Pelletshaving characteristic properties shown in Table 1 were obtained bysuitably selecting pellet preparation conditions such as the temperatureof cooling water, the shape of a die hole, take-off tension, take-offspeed, the rotation speed of the cutter blade and the temperature of thestrand at the time of cutting and conditions for selecting pellets by apunching metal or a sieve.

The pellets used in these Examples and Comparative Examples had alength, a long diameter and a short diameter of the section and astandard deviation thereof within the following ranges.

Average value of pellet lengths: 2.8 to 3.2 mm

Average value of long diameters of pellet sections: 3.0 to 3.4 mm

Average value of short diameters of pellet sections: 2.2 to 2.5 mm

Standard deviation of the above values: 0.15 mm or less

The characteristic properties of the pellets were measured by thefollowing methods. The molding of a substrate and the measurement of asilver streak were carried out by the following methods.

(1) Measurement of Bulk Density

Pellets were dropped into a SUS cylindrical vessel having a height of8.6 cm, a capacity of 100 ml and a weight of 150 g from a height of 5 cmcontinuously until they overflowed. Thereafter, the pile of the pelletswas leveled off by a plastic board, the weight of the SUS vessel filledwith the pellets was measured, and the bulk density of the pellets wascalculated from the following expression.

D=(Z−150)/100

D: bulk density (kg/l)

Z: weight (g) of a SUS vessel filled with pellets

(2) Measurement of Long Diameter/Short Diameter Ratio

The long diameters (a) and the short diameters (b) of the sections of300 pellets were measured with a micrometer to calculate the longdiameter/short diameter ratios (R). Thereafter, the average value andits standard deviation were calculated using the obtained results. Thestandard deviation was calculated based on the following expression.

Standard deviation=[{ΣR ²−(ΣR)²/300}/299]^(1/2)

R: long diameter/short diameter ratio

(3) Measurement of Weight

The weights of 300 pellets were measured with an electronic balance tocalculate the average value of weights and its standard deviation. Thestandard deviation was calculated based on the following expression.

Standard deviation=[{ΣM ²−(ΣM)²/300}/299]^(1/2)

M: weight (mg)

(4) Measurement of Air Bubble Evolution Ratio

The existence of an air bubble in 300 pellets was observed with the eyeto calculate air bubble evolution ratio.

(5) Measurement of Maximum Air Bubble Volume (Vb)

(A) Cylindrical Air Bubble (FIG. 1 and FIG. 2)

An air bubble was observed from the side of each pellet with the digitalHD microscope VH-7000 of Keyence Co., Ltd. to measure the length andwidth of the air bubble with a scale. The volume of the air bubble wascalculated from the obtained result according to the followingexpression.

Vb=π×(r/2)×(r/2)×L

Vb: volume (mm³) of air bubble

r: length (mm) in the vertical direction of air bubble

L: length (mm) in the horizontal direction of air bubble

(In FIG. 2, r is the length (mm) in a vertical direction at anintermediate point of the length L)

(B) Spherical Air Bubble (FIG. 3)

An air bubble was measured from the side of each pellet with the digitalHD microscope VH-7000 of Keyence Co., Ltd. to measure its diameter witha scale. The volume of the air bubble was calculated from the obtainedresult according to the following expression.

Vb=4/3×π×(r/2)³

Vb: volume (mm³) of air bubble

r: diameter of air bubble (mm)

(the diameter r of the air bubble in FIG. 3 is the average value ofdiameters in longitudinal and transverse directions)

The above operation was carried out on 300 pellets and the maximum valuewas taken as the maximum air bubble volume.

(6) Measurement of Air Bubble Volume Ratio

(A) Calculation of Total Volume (Vp) of Pellets

The long diameters (a) and short diameters (b) of the sections and thelengths (W) of 300 pellets were measured by means of micrometer and thetotal volume of the pellets was calculated from the followingexpression.

Vp=π×(a/2)×(b/2)×W

Vp: volume (mm³) of each pellet

a: long diameter (mm) of pellet section

b: short diameter (mm) of pellet section

W: length (mm) of each pellet

(B) Calculation of Total Volume of Air Bubbles

The total volume of air bubbles was calculated in the same manner as themaximum air bubble volume ratio.

(C) Calculation of Air Bubble Volume Ratio

The air bubble volume ratio was calculated from the following expressionusing the total volume of 300 pellets calculated in (A) and the totalvolume of air bubbles in 300 pellets which was calculated in (B).

R=(ΣVb/ΣVp×100

R: air bubble volume ratio (%)

ΣVb: total volume of air bubbles in 300 pellets

ΣVp: total volume of 300 pellets

(7) Silver Streak

A DVD mold was attached to an injection molding machine [DISK 3M III ofSumitomo Heavy Industries, Ltd.] and a nickel DVD stamper having pitswere mounted on this mold to feed pellets which were molding materialsand were dried at 120° C. for 4 hours or more by a drier to the hopperof a molding machine automatically in order to mold 300 DVD substratescontinuously at a cylinder temperature of 375° C. and a mold temperatureof 113° C. Further, these substrates were observed with the eye to checkif a silver streak was produced and the total number of silver streakswas calculated.

(8) Metering Time

A DVD mold was attached to an injection molding machine (DISK 3M III ofSumitomo Heavy Industries, Ltd.) and a nickel DVD stamper having pitswere mounted on this mold to feed pellets which were molding materialsand were dried at 120° C. for 4 hours or more by a drier to the hopperof a molding machine automatically in order to mold DVD substratescontinuously at a cylinder temperature of 373° C. and a mold temperatureof 114° C. The metering time of each molding was measured to calculatethe average value of 50 shots.

TABLE 1 Bulk density air bubble evolution maximum air bubble air bubbleweight (g) (kg/L) ratio (%) volume (mm³) volume ratio (%) Averagestandard deviation Ex. 1 0.750 27.0 1.20 0.9 17.4 1.7 Ex. 2 0.745 24.00.90 0.8 18.5 1.3 Ex. 3 0.740 23.7 0.87 0.9 18.9 1.1 Ex. 4 0.722 28.00.90 1.1 22.0 1.9 Ex. 5 0.733 24.5 0.95 0.8 20.2 1.0 C.Ex. 1 0.678 36.02.20 2.2 25.6 2.4 C.Ex. 2 0.687 37.0 2.35 2.1 25.1 2.5 C.Ex. 3 0.71333.0 1.90 1.9 23.0 2.4 C.Ex. 4 0.721 38.5 2.10 2.3 22.0 2.4 C.Ex. 50.695 39.2 2.22 2.1 24.5 2.3 long diameter/short diameter ratioproduction number of silver metering time Average standard deviationmethod streaks (seconds) Ex. 1 1.51 0.11 A 5 1.3 Ex. 2 1.45 0.12 B 2 1.3Ex. 3 1.42 0.11 A 1 1.3 Ex. 4 1.47 0.13 A 8 1.4 Ex. 5 1.38 0.15 A 9 1.4C.Ex. 1 1.45 0.16 A 80 1.6 C.Ex. 2 1.55 0.17 B 81 1.5 C.Ex. 3 1.25 0.18A 40 1.5 C.Ex. 4 1.20 0.18 A 33 1.4 C.Ex. 5 1.22 0.15 A 36 1.5 A: esterexchange method B: interfacial polymerization method Ex.: Example C.Ex.:Comparative Example

What is claimed is:
 1. A polycarbonate resin molding material foroptical use in the form of pellets of a polycarbonate resin, and having(1) a bulk density of 0.72 kg/l or more and having (2) an air bubbleevolution ratio of 35% or less, a maximum air bubble volume of 2.0 mm³or less, and an air bubble volume ratio of 2.0% or less.
 2. Thepolycarbonate resin molding material for optical use of claim 1, whereinthe pellets have (3) an average weight value of 13 to 26 mg and astandard deviation thereof of 2.2 mg or less.
 3. The polycarbonate resinmolding material for optical use of claim 2, wherein the pellets have(4) an average value of the long diameter/short diameter ratios of cutsections referred to as “long diameter/short diameter ratio” of 1.3 to1.7 and a standard deviation thereof of 0.08 to 0.15.
 4. Thepolycarbonate resin molding material for optical use of claim 1, whereinthe pellets have a bulk density of 0.73 kg/l or more.
 5. Thepolycarbonate resin molding material for optical use of claim 1 whereinthe pellets have an air bubble evolution ratio of 30% or less, a maximumair bubble volume of 1.5 mm³ or less and an air bubble volume ratio of2.0% or less.
 6. The polycarbonate resin molding material for opticaluse of claim 1, wherein the pellets have an average weight value of 14to 24 mg and a standard deviation thereof of 2.0 mg or less.
 7. Thepolycarbonate resin molding material for optical use of claim 1, whereinthe polycarbonate resin has a viscosity average molecular weight of10,000 to 22,000.
 8. The polycarbonate resin molding material foroptical use of claim 1, wherein the polycarbonate resin is apolycarbonate comprising 2,2-bis(4-hydroxphenyl)propane as a maindiphenol component.
 9. The polycarbonate resin molding material foroptical use of claim 1, where in the polycarbonate resin is apolycarbonate comprising 2,2-bis(4-hydroxyphenyl)propane as a maindiphenol component and obtained by an ester exchange method.
 10. Thepolycarbonate resin molding material for optical use of claim 1, whereinthe polycarbonate resin is a polycarbonate having a phenolic hydroxylgroup (OH group) content of 5 to 100 equivalents/ton.
 11. Thepolycarbonate resin molding material for optical use of claim 1, whereinthe pellets have (3) an average weight value of 13 to 26 mg and astandard deviation thereof of 2.2 mg or less.
 12. An optical disksubstrate formed from the polycarbonate resin molding material ofclaim
 1. 13. An optical disk substrate formed from the polycarbonatemolding material of claim
 2. 14. An optical disk substrate formed fromthe polycarbonate resin molding material of claim
 11. 15. An informationrecording medium comprising the optical disk substrate of claim
 12. 16.An information recording medium comprising the optical disk substrate ofclaim
 13. 17. An information recording medium comprising the opticaldisk substrate of claim 14.